Osmosis is a natural phenomenon whereby a solution containing low solids passes through a semi-permeable membrane into a solution having greater solids concentration. Osmotic flow ceases and reaches equilibrium when the pressure in the higher solids solution equals the osmotic pressure for the membrane. Reverse osmosis occurs when a pressure greater than the osmotic pressure forces water molecules through the semi-permeable membrane in the reverse direction.
Reverse osmosis devices are known for making potable water from sea or poluted water. The conventional reverse osmosis system consists of a pump, a reverse osmosis module and a back pressure valve. The pump supplies water to the module. The semi-permeable membrane of the module element converts 10 to 20 percent of the unpurified solution to potable water. The remaining 80 to 90 percent of the solution passes to the pressure valve, which is set to maintain a pressure in the module somewhat greater than the osmotic pressure of the overflow solution. From the back pressure valve, the solution goes to waste.
Thus, generally, the energy in overflow solution from a conventional system is lost. A number of devices, however, are known for recovering that energy. In a classic device the overflow solution impinges on a Pelton wheel attached to the pump or the pump drive motor.
Other energy recovery devices are of the energy exchange type where the energy in the overflow solution is transferred to a new solution. Generally, these devices, as in U.S. Pat. No. 3,791,768, use opposed cylinder piston pumps in which the pistons are driven by the overflow concentrate of a reverse osmosis module. The energy required to pump the portion of new solution, equal in volume to the permeated or purified water, and to overcome the friction in the system, is supplied by a mechanically-driven, auxiliary pump.
Other energy exchange devices employ a single reciprocating plunger. A recent such device is shown in U.S. Pat. No. 4,187,173. In that device, a hand lever is used for the power assist. The device includes a spool-type, three-way valve, the stem of which protrudes from the housing and is parallel with the plunger rod and is attached to the hand lever. In the downstroke, the valve stem attachment to the hand lever is the fulcrum. When the stroke of the lever reverses, the fulcrum shifts from the valve stem to the plunger rod. The plunger rod remains stationary in order to serve as the new fulcrum because of a hydraulic lock on the system.
Although useful, the device of U.S. Pat. No. 4,187,173 has a number of drawbacks. For example, during startup the hydraulic lock does not exist and priming is difficult. In addition, seals to the atmosphere are required at both ends of the valve spool thereby leading to potential for leaks and failure. Most significantly, however, is that the stroke loss due to the shifting of the three-way valve and the stroke loss due to the limited angle through which the lever may be effectively manipulated results in a larger and heavier pumping unit than should be the case and than is acceptable for many uses.